1. Field of the Invention
This invention relates to a method of producing clad steel plate having good low-temperature toughness in its as-rolled condition, in which the cladding material is a stainless steel, nickel alloy or other high alloy having good corrosion-resistance, and the base metal is a designated low alloy (low C - Mn - high Nb - trace Ti).
2. Description of the Prior Art
Considerations of safety and economic efficiency are resulting in increasing use of steel pipe clad with high alloy cladding materials such as stainless steel and nickel alloy for large-diameter pipelines used for transporting crude oil and natural gas, which contain a high level of corrosive substances such as H.sub.2 S, CO.sub.2 and Cl.sup.-.
Conventionally such pipes have been manufactured by UOE forming of rolled clad steel plate, welding the seam, and reheating and cooling (solution treatment) of the whole pipe. Recently, however, techniques have been developed that are aimed at achieving the requisite properties in the as-rolled condition, i.e. without the use of solution treatment. Such techniques are disclosed by JP-A60-216984, JP-A62-16892 and JP-A63-130283, for example.
However, with these methods it is exceedingly difficult to obtain cladding material that has good corrosion-resistance together with a base metal that has good tensile strength and low-temperature toughness. The reason for this is that, while rolling at higher temperatures (at or above 900.degree. C.) improves the corrosion-resistance of the cladding material, the low-temperature toughness of the base metal is improved by rolling at a low temperature. As a result, the need to use a lower rolling temperature in the prior art has meant that the corrosion-resistance of the clad steel has suffered.